Some of the principal goals that are sought through the increase of mechanical strength of the glass container are the following: provide greater handling safety for the manufacturer, the bottler and the user, as well as to permit the reduction of the weight of the containers (less glass quantity) especially with those containers known as "non-returnable" which need to be lighter and more resistant.
Glass traditionally has been considered to be a fragile material inspite of the theoretical calculations concerning its resistance. If we take into account that the aforementioned resistance is based on the forces that are necessary to break its atomic ties then glass does show high values of mechanical strength.
The discrepancy between the ideal strength and the one that is observed in practice is generally due to the presence of defects or imperfections on the glass surface which act as points where forces are concentrated.
These superficial defects can be generated from the time that the glass is smelted and prepared during the forming process or from the time that the glass article or specifically the glass container comes out of the forming machine until the moment that, for some reason, the container is discarded.
The condition under which the glass is naturally very strong is in the forces of pure compression.
The glass, naturally weak to tension forces can be strengthened if the surface that is subjected to tension is previously treated in such a way that residual compression forces remain after treatment. The resistance is increased in a magnitude that is equal to the forces of compression that are generated.
There exist several methods to pre-strengthen the surface of glass articles such as physical tempering, glass casing and chemical treatment, which is the one that has to do with the method of the present invention.
Under the term "Chemical Treatment" are grouped phenomena of ionic interchange both at temperature that are above as well as below the glass smelting deformation point.
I IONIC INTERCHANGE AT LOW TEMPERATURES. It is called thus because this ionic interchange has to take place at a temperature that is under the one at which glass can liberate forces. Normally this temperature is below the point where forces begin to generate due to cooling (the Strain Point), although in exceptional circumstances the interchange can take place a little above that temperature. With this method, ions that are relatively large, such as potassium or sodium, migrate to the inside of the glass in order to interchange smaller ions of sodium or lithium from the glass composition of the soda-calcium composition. Compression forces do develop at the interchange coating area due to the differential of ionic dimensions with the resulting physical inlay at the coating area. Since the temperature of the glass is less than the temperature at which these effects can take place the forces are retained to strengthen the glass.
This method is capable of developing high compression forces and consequently a high degree of strengthening of the article takes place.
The problems that have been encountered in order to apply this technology at an industrial level are the following:
Apply the treatment in a shorter time thus maintaining the reliability of the process.
Prevent the appearance of undesirable secondary effects on the container such as a chemical attack which has a result opaque or tranished glass surfaces.
Make the article to be resistant after treatment inspite of the fact that the glass article was damaged before it was treated.
In order to overcome these problems several application methods of the ionic interchange media have been proposed.
Weber and others, in their U.S. Pat. No. 3,218,220, suggested the immersion of the article in a mixture of smelted salts (which therefore, has to be effected outside of the production line) while the article is at an ionic interchange temperature, as well as the use of an electrolyte. However, this method is difficult to put into practice because up to now the problem of how to prevent thermic shock at the time the potassium salts are applied to the hot bottles has not been resolved. This problem is discussed in pages 339 and 340 of the article entitled "Methods of Influencing Glass Container Strength" published by D. G. Osborne in the September issue of the 1982 "Glass" magazine.
The application of the salts in an aqueous solution is easier to implement. However, if they are applied when the article is near its deformation point glass cracks are generated and some salts undergo hydrolysis which produce compounds that attack the container.
The application of the salts in a powder form prevents the problems of cracks and hydrolisis but this method produces inefficiency with respect to the quality of ionic interchange. This is so because there is not good adherence of the powder salts to the article.
The salts have also been applied in an aqueous solution or dissolved in an organic liquid at low temperatures as is suggested by Grubb and others in their U.S. Pat. No. 3,844,754.
The solution can be sprayed on the container at moderately high temperatures so that one or more of the liquid components are evaporated and a solid film on the glass surface is formed. Later the glass article is heated to a treating temperature.
Levene in his U.S. Pat. Nos. 3,853,673 and 3,853,674 proposes the formation of a gel coating at a high temperature where the gel contains the salts or salts used for the ionic interchange.
Watanabe in his U.S. Pat. Nos. 4,021,218 and 4,206,253 emphasizes the use of a high temperature decomposition surfactant that also has an excellent miscibility with the potassium salts used. He applies an aqueous solution to the article at room temperature while the solution is from 30.degree. to 75.degree. C. hotter. As a result a coating of potassium salts is obtained on the glass surface.
The problem presented with these methods resides in that since they are all carried out at low temperatures they have to be applied out of the production line. In other words, when the containers have a temperature which is different from that of the production line - previously and/or after the salt application-it is necessary to subject the glass articles to a new thermic treatment in order to obtain the desired ionic interchange and the strengthening of the containers. Consequently this represents a method that takes longer and is not very economic.
II. IONIC INTERCHANGE AT HIGH TEMPERATURES. The development of this technique for hardening glass articles was accidental. This happend when at the annealing lehrs the combustion gases contained SO.sub.2 in quantities that permitted the reaction with the alkali on the surface of the glass to produce Na.sub.2 SO.sub.4. The articles came out of the lehr with a white coating which was removed by washing. The surface coating, having a different composition, had a minor coefficient of expansion, which gave upon cooling residual compression forces.
Another procedure is described by Hood and others in their U.S. Pat. No. 2,779,136 wherein a glass that contains interchangeable potassium and/or sodium ions is treated at a temperature that is higher than the point where forces begin to generate due to cooling (the Strain Point) with a lithium ion source. For example, a smelted lithium salt. The lithium ions migrate to the inside of the glass in order to interchange ions of sodium or potassium. The interchanged coating suffers a molecular rearrangement in order to accomodate the small lithium ions and forms a glass coating with a lesser expansion coefficient than the one of the original glass. When the article cools compression forces develop in the new glass coating due to the expansion coefficient differences.
In the U.S. Pat. No. 2,075,466 ions of copper or silver are replaced by sodium ions.
The objective of this process is the formation of a glass coating with a different composition than the one of the original glass and with a minor expansion coefficient.
The results are similar to these obtained with a physical tempering when the interchange is optimum, except when a crystalline stage appears on the glass surface. However, with a physical tempering, which is practically impossible to achieve with bottles because of their irregular shapes it can be achieved in minutes with the method proposed. This different form the ionic interchange physical tempering of the process which can take up to several hours to be complete.
Another treatment which offers the possibility of application in the production line is the one claimed by the U.S. Pat. No. 3,791,809 of law. This patent describes a method used to strengthen glass articles through the use of ionic interchange between sodium and potassium. This is done by applying pulverized salts of potassium nitrate and tripotassium phosphate to the glass when they are still hot. The result is that one of the salts fuses over the article in order to retain the salt of ionic interchange and the articles are heated thereafter at high temperatures in order to carry out the ionic interchange reaction. Lastly the glass artcile is cooled gradually to room temperature.
The said procedure, even though it has the advantage of being able to apply it at the production line, it has disadvantage of requiring a previous and/or thermic treatment in order to obtain fusion of the salts on the article and a relatively uniform ionic interchange on the glass surface.
Searching to solve the problems of the previous methods mentioned above, the inventor of the present invention found that in order to obtain shorter treatment times and maintain the reliablity of the method it was necessary that the same be applied at some place on the production line. It was also found that in order to avoid undersirably secondary effects the treatment had to be made in such a way that it would not leave traces or visible coatings of compounds that would take away the brillance from the containers. Also in order to make the containers resistant after the treatment, inspite of having been damaged previously, the method used must be of such a type that the results had to be integrated within the containers and not only as a mere coating that, when lost, the resistance would also be lost. The results had to guarantee the uniformity of forces on the glass surface with greater safety that would be obtained with a simple previous or posterior heat treatment.
Thus the inventor of the present invention was able to obtain the method that will strengthen glass articles through an ionic interchange so that the strengthening treatment is in fact intimately integrated with the article in an electrostatic form. This makes easier the application and the obtainal of the ionic interchange and consequently the uniformity of the forces generated on the surface of the glass and the reliability of the method is guaranteed. The method described includes then the use of spraying a flux of ionic interchange powders electrostatically charged onto the glass articles at the temperature that they have after having been formed and before entering the annealing lehr (or gradual cooling). In other words, at the production line, so that the treatment could be carried out within the same production times without there beging any additional delays for the treatment. Thus all of the problems presented previously by others methods were solved.